MASSPROP Gives Incorrect Volume in World UCS

[A. Blundon]

I have a 3D model in which the MASSPROP command gives the incorrect volume when in the world UCS. If I change the USC by rotating about Y 90° the volume comes out correct.

I tried copy and pasting the model into a new file.
I tried exporting to an ACIS and then reimporting.

Nothing seems to work. Volume is not UCS dependent, unlike say the Centroid or Moment of Inertia.

Any ideas?